/Engine/Plugins/Online/OnlineSubsystemUtils/Source/OnlineSubsystemUtils/Public/VoiceEngineImpl.h
This can be used to get how loud the given microphone input is, typically on a scale between 0.0 and 1.0.
-1.0 if getting the mic amplitude is not supported on this platform or LocalUserNum was invalid, 0.0-1.0 otherwise.
virtual float GetMicrophoneAmplitude ( int32 LocalUserNum )